Electrochemical cells dual electrode

The combination of electrochemistry and photochemistry is a fonn of dual-activation process. Evidence for a photochemical effect in addition to an electrochemical one is nonnally seen m the fonn of photocurrent, which is extra current that flows in the presence of light [, 89 and 90]. In photoelectrochemistry, light is absorbed into the electrode (typically a semiconductor) and this can induce changes in the electrode s conduction properties, thus altering its electrochemical activity. Alternatively, the light is absorbed in solution by electroactive molecules or their reduced/oxidized products inducing photochemical reactions or modifications of the electrode reaction. In the latter case electrochemical cells (RDE or chaimel-flow cells) are constmcted to allow irradiation of the electrode area with UV/VIS light to excite species involved in electrochemical processes and thus promote fiirther reactions. 

The first combined HPLC-electrochemical measurements of vitamin K used the reductive mode, but this technique suffered from interference from the reduction of oxygen. A redox method was later developed that eliminated this interference, and provided a 10-fold increase in sensitivity over photometric detection and an improved selectivity. The coulometric detector employed in the redox mode is equipped with a dual-electrode cell in which phylloquinone is first reduced upstream at the generator electrode and the hydroquinone is reoxidized downstream at the detector electrode. 

Eluted peaks were detected by electrochemical oxidation using the ESA 5100A coulometric detector equipped with an ESA 5010 dual electrode detector cell and a guard cell (ESA, Bedford, MA). The guard cell was placed between the pump and injector (19) and set at a potential of 0.75V. The first electrode of the analytical cell was set at a potential of 0.5V and the second electrode at which OA and N-acetyl OA are oxidized was set at 0.7V. 

Based on these electrochemical studies we developed a method for the quantitation of ajmalicine and catharanthine in cell cultures. These alkaloids were extracted from freeze-dried cells and purified by the solid-phase procedure described by Morris et al. (1985), except that ethanol was used as the extracting solvent instead of methanol. A dual-electrode coulometric cell was used in the screen mode. The potential of the first electode was set at +0.2 V (vs. Pd), which was at the base of catharanthine s voltammogram. The alkaloids were detected by the second electrode at +0.8 V, as this offered the best S/N ratio. Higher potentials led to lower S/N ratio, since the background current and noise started to increase exponentially above +0.85 V, due to the oxidation of water. The mobile phase was purified by a guard cell between the pump and injector. The guard cell operated at +0.8V. 

A simple two-electrode electrochemical cell consisting of either single or dual polarizable electrode(s) is normally required for amperometric titrations of various organic and inorganic substances. By definition, a polarizable electrode is a suitable electronic conductor whose potential changes even with the passage of relatively small current. In contrast, the potential of a nonpolarizable electrode, such as the saturated calomel and silver - silver chloride electrodes that are commonly employed as reference electrodes, remains reasonably constant even when a large current is passed. 

Scanning electrochemical cell microscopy (SECCM) uses a capillary with double compartment (dual barrel capillary) pulled to a sharp opening (Fig. 13b) as scanning probe [36]. Both compartments of the capillary are filled with the electrolyte solution, and a quasi-reference counter electrode (QRCE) is inserted in each compartment (Fig. 13a). 

In a similar way, electrochemistry may provide an atomic level control over the deposit, using electric potential (rather than temperature) to restrict deposition of elements. A surface electrochemical reaction limited in this manner is merely underpotential deposition (UPD see Sect. 4.3 for a detailed discussion). In ECALE, thin films of chemical compounds are formed, an atomic layer at a time, by using UPD, in a cycle thus, the formation of a binary compound involves the oxidative UPD of one element and the reductive UPD of another. The potential for the former should be negative of that used for the latter in order for the deposit to remain stable while the other component elements are being deposited. Practically, this sequential deposition is implemented by using a dual bath system or a flow cell, so as to alternately expose an electrode surface to different electrolytes. When conditions are well defined, the electrolytic layers are prone to grow two dimensionally rather than three dimensionally. ECALE requires the definition of precise experimental conditions, such as potentials, reactants, concentration, pH, charge-time, which are strictly dependent on the particular compound one wants to form, and the substrate as well. The problems with this technique are that the electrode is required to be rinsed after each UPD deposition, which may result in loss of potential control, deposit reproducibility problems, and waste of time and solution. Automated deposition systems have been developed as an attempt to overcome these problems. 

An interesting idea has been to prepare the photosensitive electrode on site having the liquid play the dual role of a medium for anodic film growth on a metal electrode and a potential-determining redox electrolyte in the electrochemical solar cell. Such integration of the preparation process with PEC realization was demonstrated initially by Miller and Heller [86], who showed that photosensitive sulfide layers could be grown on bismuth and cadmium electrodes in solutions of sodium polysulfide and then used in situ as photoanodes driving the... 

Aniline, methyl aniline, 1-naphthylamine, and diphenylamine at trace levels were determined using this technique and electrochemical detection. Two electrochemical detectors (a thin-layer, dual glassy-carbon electrode cell and a dual porous electrode system) were compared. The electrochemical behavior of the compounds was investigated using hydrodynamic and cyclic voltammetry. Detection limits of 15 and 1.5nmol/l were achieved using colourimetric and amperometric cells, respectively, when using an in-line preconcentration step. 

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